Asymmetric band-gap engineered nonvolatile memory device

ABSTRACT

Systems and methods are provided for nonvolatile memory devices that incorporate a band-gap engineered gate stack with asymmetric tunnel barriers. One embodiment of a memory device includes first and second source/drain regions separated by a channel region in a substrate, a control gate, and a gate stack between the control gate and the channel region. The gate stack includes a first insulator region in contact with the channel region, a floating charge-storage region in contact with the first insulator region, and a second insulator region in contact with the floating charge-storage region and the control gate. The gate stack includes selected material, in conjunction with control gate metallurgy, for providing desired asymmetric energy barriers that are adapted to primarily restrict carrier flow during programming to a selected carrier between the control gate and the floating charge-storage region, and to retain a programmed charge in the floating charge-storage region. Other aspects are provided herein.

Cross Reference To Related Applications

This application is a Divisional of U.S. application Ser. No.10/075,484, filed Feb. 12, 2002 which is incorporated herein byreference.

This application is related to the following co-pending, commonlyassigned U.S. patent applications which are herein incorporated byreference in their entirety: “Flash Memory With Low Tunnel BarrierInterpoly Insulators,” Ser. No. 09/945,507, filed on Aug. 30, 2001;“Programmable Array Logic Or Memory Devices With Asymmetrical TunnelBarriers,” Ser. No. 09/943,134, filed on Aug. 30, 2001; and “ScalableFlash/NV Structures and Devices With Extended Endurance,” Ser. No.09/944,985, filed on Aug. 30, 2001.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to integrated circuits and, moreparticularly, to nonvolatile programmable memory cells.

BACKGROUND OF THE INVENTION

Conventional nonvolatile flash and Electronically Erasable ProgrammableRead Only Memory (EEPROM) devices are “dual-threshold” Field EffectTransistor (FET) devices. These devices include a floating silicon gate(“floating gate”) isolated from the top programming gate (“controlgate”). These devices are programmed (write and/or erase) by applying aset of programming potentials between the control gate and the siliconsubstrate. Devices are read as being either in the nonconducting stateor in the conducting state based on the threshold state of the FETdevice. Hot carriers, which are usually electrons, are conventionallysupplied and injected from the silicon substrate. Electrons arecollected by the floating gate to raise the threshold to thenonconducting state for an NFET device and conversely discharged intothe substrate to lower the threshold and return to the conducting state.While the reading of the states is similar to the reading of DynamicRandom Access Memory (DRAM) devices, the writing or erasing processtypically takes tens or hundreds of milliseconds. Therefore, it isdesirable to improve the erase and/or write speed to enhance the rangeof applications for nonvolatile devices.

The conventional nonvolatile devices discussed above involve high energy(“hot”) charge transport between the substrate and the floating gate.Part of the energy of the hot carriers is transferred to the interfacelattice between the silicon substrate and the gate oxide. As a result,interface bonds break and interface characteristics get degraded aftermultiple write-erase cycles. The term “endurance” relates to theseeffects that multiple write-erase cycles have on the device.Consequently, the hot charge transport generates surface states at thesilicon-oxide interface and creates local weak spots in the gate oxidethat negatively affects the device by degrading the FET transconductance(Gm), enhancing the stored charge loss from the floating gate (i.e.retention loss), and reducing the endurance (i.e. operable write-erasecycles) of the device.

It has been proposed to preserve the integrity of the silicon-oxideinterface by providing primary charge carrier transport between thefloating gate and the control gate. For example, it has been proposed toappropriately design the gate stack such that the charge transport takesplace preferentially and primarily between the control gate and thefloating gate by field emissions of carriers from either or both thecontrol gate and the floating gate during write and erase operations.The nonvolatile devices include a control gate/floating gate capacitorand a floating gate/substrate capacitor. A programming voltage iscapacitively divided between these two capacitors. Therefore, in orderto provide the primary charge carrier transport between the floatinggate and the control gate, the gate stack is designed so that more ofthe potential, and thus more of the electric field, is imposed betweenthe control gate and the floating gate rather than between the floatinggate and the substrate. Such a device enters a low threshold state byhole transport from the control gate and subsequent collection at thefloating gate, or by electron transport from the floating gate throughthe dielectric layer and discharge to the control gate, or by acombination of both the hole transport and the electron transport asstated above. Conversely, such a device enters a high threshold state byhole transport from the floating gate and subsequent discharge to thecontrol gate, or by electron transport from the control gate through thedielectric layer and collection into the floating gate, or by acombination of both the hole transport and the electron transport asstated above. It is noted that the mechanism for charge transport forsuch devices is based on field emission alone. Conventional devicesoperate by multiple mechanisms for charge transport which include hotelectron emission from the device channel as well as electron fieldemission from the substrate for a write operation; whereas for an eraseoperation, the mechanisms may include avalanche hole emission from thedevice junction and/or electron field emission from the control gate.

Carrier field emission (or tunneling) is exponentially dependent on thefield in the insulator and the potential barrier height for emissioninto the insulator, as provided by the following equation:$\frac{J}{E^{2}} \propto {{\mathbb{e}}^{\frac{- {(\Phi_{B})}^{\frac{3}{2}}}{E}}.}$For the above equation, J represents current density, E represents theelectric field, and Φ_(B) represents the barrier height or energybarrier. The carrier transport is capable of being significantlyenhanced by increasing the electric field (E) imposed across theinsulator and is capable of being significantly reduced by obtaining alarger energy barrier height (Φ_(B)) between materials.

Conventional nonvolatile devices may be termed“substrate-sourced-carrier devices” (SSCD) and the non-conventionalnonvolatile devices discussed above may be termed “gate-sourced-carrierdevice” (GSCD). Since the floating gate acts as a capacitive voltagedivider, the potential imposed between the control gate and thesubstrate is divided between the control gate/floating gate capacitorand the floating gate/substrate capacitor. SSCD devices are designed tohave higher coupling constant (K>0.5) to achieve a higher potential dropbetween the floating gate and the substrate to facilitate chargeinjection from the substrate during a write or erase operation. GSCDsare designed to have lower coupling constants (K<0.5) for a greaterpotential drop between the control gate and the floating gate for suchoperation. With the smaller coupling constant, GSCDs are able to have asmaller geometry and are able to provide higher cell and chip densitythan conventional SSCDs. However, disadvantages with GSCDs include dataretention and read disturb problems as explained below.

With respect to the data retention problem, the GSCD is designed tofacilitate charge transport between the control gate and the floatinggate. The built-in field between the floating gate and the control gateis higher when the charge (electrons or holes) is stored in the floatinggate. If the barrier height for carrier emission is sufficiently low,charge is transported more easily between the floating gate and thecontrol gate, resulting in an enhanced stored charge loss, poorer dataretention and loss of non-volatility.

With respect to read disturb problems, all bits from the same word linecolumn are subjected to the read-potential between the control gate andthe substrate whenever a specific bit is read. The read-potential ispositive at the control gate for NFETs. The addition of the readpotential to the built-in potential enhances the stored charge loss(electrons to the control gate and holes to the substrate) during theread-pulse period for all those other bits on the word line holdingcharge in their floating gates. Sufficient charge is capable of beinglost due to these read pulses over time to cause permanent loss of dataunless the data is periodically refreshed.

The data retention problem and read disturb problem have preventedapplications of GSCD in the past. These problems are capable of beingdesigned out of the conventional SSCD by applying an oxide insulator atthe floating gate-substrate interface with a high barrier height of 3.2ev, by applying a thicker oxide-nitride-oxide (ONO) layer on the top andside of the floating gate such that the equivalent oxide thickness(t_(ox.eq.)) between the control gate and the floating gate is greaterthan 70 nm, and by selecting the cell geometry (by enlarging the cellsize) to achieve a larger coupling ratio (K>0.5). For GSCD devices, suchapproaches with K<0.5 require higher write/erase voltages and result inslower write/erase speeds. Any attempt to improve speed by reducing theinsulator thickness between the control gate and the floating gate orlowering the barrier height of the insulator between the control gateand the floating gate enhances the data retention and read disturbproblems such that a frequent refresh is needed to prevent data loss.

Therefore, there is a need in the art to provide an improved GSCD withimproved voltage scalability, density and endurance while maintainingthe smaller geometry and the resulting higher cell and chip densityassociated with GSCD.

SUMMARY OF THE INVENTION

The above mentioned problems are addressed by the present subject matterand will be understood by reading and studying the followingspecification. The present subject matter provides nonvolatile memorydevices that incorporate a band-gap engineered gate stack withasymmetric tunnel barriers. The gate stack is designed with appropriateenergy barriers such that the memory device is capable of beingprogrammed (i.e. written and erased) primarily through charge transportbetween the control gate and the floating charge-storage medium (i.e.floating gate or floating plate). The gate stack is further designedwith appropriate energy barriers to retain the stored charge. The gatestack is further designed with appropriate stack geometry to furtherlimit carrier flow from and to the substrate by providing theappropriate capacitive coupling (k<0.5).

The gate stack materials are selected with the appropriate energybarrier between the silicon substrate, the floating gate/plate, thecontrol gate, and the respective interface insulators such that fieldemission is selective to either electrons or holes. While the selectedcarrier transport is enhanced between the control gate to the floatinggate/plate due to the reduced barrier height at the controlgate-insulator interface during programming, charges are retained in thefloating gate/plate due to relatively higher barrier energy at thefloating gate-insulator interface caused by the band-gap asymmetry. Theinsulator at the silicon substrate interface is selected so as toprovide large barrier heights for both electrons and holes for minimalemission of carriers from the silicon substrate during write or erase.Additionally, the insulator thickness and stack geometry is designed toprovide capacitive coupling K<0.5 to further limit carrier flow from andto the silicon substrate. In this manner, carrier flow is restrictedprimarily between the control gate and the floating gate/plate in eitherdirection for both write and erase.

One aspect of the present subject matter is a nonvolatile memory device.One embodiment of the memory device includes first and secondsource/drain regions separated by a channel region in a substrate, acontrol gate, and a gate stack between the control gate and the channelregion. The gate stack includes a first insulator region in contact withthe channel region, a floating charge-storage region in contact with thefirst insulator region, and a second insulator region in contact withthe floating charge-storage region and the control gate. The gate stackincludes selected material for providing desired asymmetric energybarriers that are adapted to primarily restrict carrier flow duringprogramming to a selected carrier between the control gate and thefloating charge-storage region, and to retain a programmed charge in thefloating charge-storage region.

One embodiment of the memory device includes a p-substrate. A first ntype source/drain region and a second n type source/drain region areseparated by a channel region in the substrate. A Silicon Dioxide (SiO₂)layer is in contact with the channel region, and a Tantalum Oxide(Ta₂O₅) layer is in contact with the SiO₂ layer. A charge-storage regionis in contact with the Ta₂O₅ layer, and a Zirconium Oxide (ZrO₂) layeris in contact with the charge-storage layer. An aluminum control gate isin contact with the ZrO₂ layer. According to one embodiment, thecharge-storage region includes a silicon floating gate. According toanother embodiment, the charge-storage region includes a floating platecontaining silicon nano crystals to facilitate field emission and chargestorage.

These and other aspects, embodiments, advantages, and features willbecome apparent from the following description of the invention and thereferenced drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a floating gate embodiment of thenonvolatile device of the present invention.

FIG. 2 is a band diagram illustrating an asymmetric band-gap gate stackincorporated in the device of FIG. 1.

FIG. 3 illustrates charge transport for an Erase Process, defined as atransition from a nonconducting high threshold (1) to a conducting lowthreshold (0), using the band diagram of FIG. 2.

FIG. 4 illustrates charge transport for a Write Process defined as atransition from a conducting low threshold (0) to a nonconducting highthreshold (1), using the band diagram of FIG. 2.

FIG. 5 illustrates operating conditions for erasing an NFET nonvolatiledevice of FIG. 1.

FIG. 6 illustrates operating conditions for writing an NFET nonvolatiledevice of FIG. 1.

FIG. 7 illustrates operating conditions for reading an NFET nonvolatiledevice of FIG. 1.

FIG. 8 illustrates one embodiment of a floating plate embodiment of thenonvolatile device of the present invention.

FIG. 9 is a graph showing refractive index of silicon-rich siliconnitride films versus SiH₂Cl₂/NH₃ flow rate ratio.

FIG. 10 is a graph showing current density versus applied field forsilicon-rich silicon nitride films having different percentages ofexcess silicon.

FIG. 11 is a graph showing flat band shift versus time at an appliedfield of 4×10⁶ volts/cm for silicon-rich silicon nitride films havingvarying percentages of excess silicon.

FIG. 12 is a graph showing flat band shift versus time at an appliedfield of 7×10⁶ volts/cm for silicon-rich silicon nitride films havingvarying percentages of excess silicon.

FIG. 13 is a graph showing apparent dielectric constant K versusrefractive index for both Silicon Rich Nitride (SRN) and Silicon RichOxide (SRO).

FIG. 14 is a cross-section view of a conventional nonvolatile fieldeffect transistor (NV FET) device.

FIG. 15 illustrates the capacitive coupling for a conventional NV FETdevice.

FIG. 16 illustrates the capacitive coupling for a nonvolatile floatingplate device.

FIG. 17 illustrates the average field enhancement due to theincorporation of a top injection layer in a gate stack for a nonvolatilefloating plate device.

FIG. 18 illustrates the average field enhancement due to theincorporation of a bottom injection layer in a gate stack for anonvolatile floating plate device.

FIG. 19 illustrates the average field enhancement due to theincorporation of both a top injection layer and a bottom injection layerin a gate stack for a nonvolatile floating plate device.

FIG. 20 illustrates one memory array embodiment according to the presentinvention.

FIG. 21 is a simplified block diagram of a high-level organization of anelectronic system according to the teachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention refers to theaccompanying drawings which show, by way of illustration, specificaspects and embodiments in which the invention may be practiced. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural, logical,and electrical changes may be made without departing from the scope ofthe present invention. In the following description, the terms wafer andsubstrate are interchangeably used to refer generally to any structureon which integrated circuits are formed, and also to such structuresduring various stages of integrated circuit fabrication. Both termsinclude doped and undoped semiconductors, epitaxial layers of asemiconductor on a supporting semiconductor or insulating material,combinations of such layers, as well as other such structures that areknown in the art. The term “horizontal” as used in this application isdefined as a plane parallel to the conventional plane or surface of awafer or substrate, regardless of the orientation of the wafer orsubstrate. The term “vertical” refers to a direction perpendicular tothe horizontal as defined above. Prepositions, such as “on”, “side” (asin sidewall), “higher”, “lower”, “over” and “under” are defined withrespect to the conventional plane or surface being on the top surface ofthe wafer or substrate, regardless of the orientation of the wafer orsubstrate. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined only by the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The present subject matter provides nonvolatile memory devices thatincorporate a band-gap engineered gate stack with asymmetric tunnelbarriers. The gate stack is designed with appropriate energy barriers,in conjunction with a predetermined control gate metallurgy, such thatthe memory device is capable of being programmed primarily throughcharge transport between the control gate and the floatingcharge-storage medium (i.e. floating gate or floating plate) using fieldemissions that are selective either to electrons or holes. The gatestack is further designed with appropriate energy barriers to retain thestored charge. The gate stack is further designed with appropriate stackgeometry to further limit carrier flow from and to the substrate byproviding the appropriate capacitive coupling (k<0.5). The gate stack isfurther designed with material selected and combined to significantlyenhance field emission at a reduced field.

FIG. 1 illustrates one embodiment of a floating gate embodiment of thenonvolatile device of the present invention. The device 100 is formed ona substrate 102. In one embodiment, the substrate 102 includesp—silicon. A first source/drain region 104 and a second source/drainregion 106 are formed in the substrate 102 between isolation regions108. The source/drain regions 104 and 106 are separated by a channelregion 110 of the substrate 102. In one embodiment, each of thesource/drain regions 104 and 106 include n−silicon positioned to definethe channel region 110 in the substrate 102, and n+silicon used toprovide a drain contact and a source contact.

An asymmetric gate stack 112 is formed over the channel region 110 ofthe substrate 102. The gate stack 112 separates a control gate 114 fromthe substrate 102. According to one embodiment, the control gate 114includes aluminum, and is connected to an aluminum word line. Oneembodiment of the asymmetric gate stack 112 includes a first set ofinsulator region 116, a conventional silicon floating gate 118 thatfunctions as a floating charge-storage region, and a second insulatorregion 120. The gate stack 112 is engineered to provide the desiredasymmetric barrier energies for charge transport between the three nodes(substrate, floating charge-storage region, and control gate) of thedevice, and is further engineered to provide the desired electric fieldsbetween the three nodes of the device.

In one embodiment of the asymmetric gate stack, the first insulatorregion 116 includes a layer of Silicon Dioxide (SiO₂) 122 disposed overthe channel region, and a layer of Tantalum Oxide (Ta₂O₅) 124 disposedover the layer of SiO₂ 122. One embodiment of the layer of SiO₂ 122includes approximately 2 nm of NH₃-treated SiO₂. Another embodiment ofthe layer of SiO₂ 122 includes approximately 2 nm of NO-treated SiO₂. Inone embodiment, the layer of Ta₂O₅ 124 has an equivalent oxide thickness(t_(ox.eq.)) of approximately 3 to 5 nm. In one embodiment, the layer ofTa₂O₅, has an equivalent oxide thickness (t_(ox.eq.)) of approximately 4nm. The silicon floating gate 118 is disposed on the layer of Ta₂O₅ 124.Conventional silicon floating gates may be approximately 1,000 to 2,000Å (100 to 200 nm) thick. The invention is not so limited, however, toany particular dimension of the silicon floating gate. A layer ofZirconium Oxide (ZrO₂) 126 is disposed on the silicon floating gate. Inone embodiment, the layer of ZrO₂ 126 has an equivalent oxide thickness(t_(ox.eq.)) of approximately 3-5 nm. An Oxide-Nitride-Oxide (ONO)sidewall 128 surrounds the asymmetric gate stack and isolates the devicefrom other integrated circuit devices.

It is noted that the nonvolatile device is capable of being viewed asthree nodes that define two capacitors. The three nodes include thecontrol gate, the floating gate, and the substrate. The two capacitorsinclude the control gate/floating gate capacitor and the floatinggate/substrate capacitor. Programming potentials are capable of beingapplied to the control gate and the substrate to produce desiredelectric fields, i.e. desired electromotive force (EMF), to allow forthe desired transport of charge (holes and/or electrons) between thevarious nodes of the device. Further, it is noted that charge (holes orelectrons) is capable of being accumulated and stored on the floatinggate. The charge on the floating gate provides a built-in potential thateither raises or lowers the threshold voltage of the device. In an NFETdevice, for example, accumulated holes reduce the threshold voltage ofthe device so that the device is in a conducting state, and accumulatedelectrons increase the threshold voltage of the device so that thedevice is in a nonconducting state.

FIG. 2 is a band diagram illustrating one embodiment of an asymmetricband-gap gate stack incorporated in the device of FIG. 1. The banddiagram is useful for illustrating the charge transport and chargestorage during device operation. The band diagram includesrepresentations for the silicon substrate 202, the aluminum control gate214, and the asymmetric gate stack 212 between the substrate and thecontrol gate. One embodiment of the asymmetric gate stack 212 includes afirst insulator region 216 that includes ˜2 nm of SiO₂ 222 and ˜3-5 nmt_(ox.eq.) of Ta₂O₅ 224, a silicon floating gate 218, and a secondinsulator region 220 that includes˜3-5 nm t_(ox eq.) of ZrO₂ 226.

The band diagram of FIG. 2 shows various energy barriers between thematerials. From the viewpoint of the silicon substrate 202, for example,electrons encounter a 3.2 ev energy barrier and holes encounter a 4.7 evenergy barrier attributable to the SiO₂ 222 when they are transportedfrom the silicon substrate 202 to the silicon floating gate 218. Fromthe viewpoint of the control gate 214, electrons encounter a 3.8 evenergy barrier and holes encounter a 1.9 ev energy barrier attributableto the ZrO₂ 226 when they are transported from the aluminum control gate214 to the silicon floating gate 218. From the view point of thefloating gate 218, electrons encounter a 1.5 ev energy barrier and holesencounter a 3.1 ev energy barrier attributable to the ZrO₂ 226 when theyare transported from the floating gate 218 to the control gate 214.Additionally, electrons encounter a 1.0 ev energy barrier and holesencounter a 2.9 ev energy barrier attributable to the Ta₂O₅ 224 as wellas electrons encountering an additional 2.2 ev energy barrier and holeencountering an additional 1.8 ev attributable to the SiO₂ when they aretransported from the floating gate toward the substrate. It is notedthat electrons exhibit finite probability of directly tunneling from theTa₂O₅ 224 through the SiO₂ 222 to the substrate 202. However, due tolower mobility, holes exhibit reduced probability to directly tunnelfrom the Ta₂O₅ 224 through the SiO₂ 222 to the substrate 202 compared tothe electrons. In any event, the SiO₂ 222 provides an additional barrierto charge transport off of the floating gate 218 toward the substrate202.

Programming voltages are capable of being applied to the siliconsubstrate 202 and the control gate 214 to provide sufficient fieldstrength for achieving the desired charge transport. FIGS. 3 and 4illustrate the charge transport that occurs when these programmingvoltages are applied. For simplicity, field induced band bending has notbeen shown. One should be mindful of the following points when viewingthese diagrams. The floating gate 218 is capable of storing charge so asto provide a built-in potential between the floating gate 218 and thesubstrate 202 and between the floating gate 218 and the control gate214. This built-in potential is capable of affecting the chargetransport during voltage transients that occur when the programmingvoltage is first applied or switched. Further, positive and negativeprogramming voltages have a large magnitude. In an NFET device which hasa p-substrate, negative programming voltages are applied to thesubstrate 202 so that the PN junctions of the device are reversed biased(within the tolerance of the junction), and positive programmingvoltages are applied to the control gate 214. Also, it is possible toengineer the asymmetric gate stack geometry (for example capacitivearea), and in particular to vary the equivalent oxide thicknesses of thefirst insulator region and the second insulator region, to modify thecapacitive divider effects between the control gate/floating gatecapacitor and the floating gate/substrate capacitor. As such, the gatestack 212 is capable of being engineered to apply the majority of theEMF that is associated with the applied programming voltage across thesecond insulator region disposed between the control gate 214 and thefloating gate 218 rather than across the first insulator region disposedbetween the floating gate 218 and the substrate 202. Higher EMFs providehigher energy to the charges so as to promote charge emission andtransport.

FIG. 3 illustrates charge transport for an Erase Process, defined as atransition from a nonconducting high threshold (1) to a conducting lowthreshold (0), using the band diagram of FIG. 2. Thicker arrowsrepresent primary charge transport. During the erase process, a +12Vprogramming voltage is applied to the control gate, and the substrate isgrounded to 0 V as shown in FIG. 5. The primary charge transportinvolves the emission and transport of holes, designated as 330, fromthe aluminum control gate through the ZrO₂ and to the silicon floatinggate because of the low 1.9 ev hole barrier height from the aluminumcontrol gate to the ZrO₂ of the second insulator region and because ofthe comparatively higher EMF across the floating gate/control gatecapacitor as compared to the silicon substrate/floating gate capacitorpart of the gate stack. These holes 330 tunnel through the ZrO₂ and arecollected at the floating gate. Holes are not likely to be injected andtransported from the silicon floating gate because of the high holeenergy barriers associated with the charge transport from the siliconfloating gate through the Ta₂O₅ and the SiO₂ to the silicon substrate.The collection of holes at the floating gate lowers the thresholdvoltage of the NFET device and programs the device to a conducting or“0” state. That is, the NFET device is erased. At the same time, acomplimentary charge transport occurs as electrons, designated as 332,are emitted and transported from the silicon floating gate through theZrO₂ to the aluminum control gate because of the low 1.5 ev electronbarrier height from the silicon floating gate to the ZrO₂ and because ofthe comparatively higher EMF across the floating gate/control gatecapacitor as stated above. Together, hole trapping and electrondetrapping at the floating gate enhance the erase process.

Two additional charge emission and transport processes are alsoidentified in FIG. 3. Electron emission and transport from the siliconsubstrate is identified as 334 while hole emission and transport fromthe floating gate is identified as 336. However, such mechanisms aredramatically suppressed due to large barrier energies (3.2 ev forelectrons and 4.7 ev (2.9 ev+1.8 ev) for holes), and reduced programmingfield (E) between silicon substrate and the floating gate (k<0.5 bydesign) as discussed with the carrier field emission equation previouslyprovided in the section entitled Background of the Invention. Thesesmaller charge transports 334 and 336 are generally represented bythinner arrows.

FIG. 4 illustrates charge transport for a Write Process defined as atransition from a conducting low threshold (0) to a nonconducting highthreshold (1), using the band diagram of FIG. 2. During the writeprocess, a +12 V programming voltage is applied to the substrate via thedrain diffusion as shown in FIG. 6, while the control gate is groundedto 0V. It is noted that the programming voltage is brought into thespecific cell via its drain (n+) diffusion while the substrate is madeto float. During such process, the channel of the specific cell getscapacitively coupled and rises to the same potential as the drain. Inone embodiment, if a block of cells to be written simultaneously, therelevant substrate block and drain nodes are held together during suchwriting process. Prior to the writing onset, the specific cell holds abuilt-in positive potential at the floating gate due to the trappedholes (being in the erased conducting state). During writing, thesubstrate potential rises to +12 V and with coupling coefficient of thecell designed to be k<0.5, more than half of the 12 Volts is imposedbetween the floating gate and the control gate. This potential isfurther enhanced by the built-in potential of the holes in the floatinggate. As a result, the field (EMF) between the floating gate and thecontrol gate attains significantly higher value as compared to thatbetween the substrate and the floating gate at the onset of the writingprocess. This high field induces hole emission and transport 436 fromthe floating gate to the control gate. Simultaneously, this high fieldalso induces complementary electron injection and transport 438 from thecontrol gate to the floating gate as shown in FIG. 4. Mechanism 436involves the positive charge removal from the floating gate whilemechanism 438 involves positive charge neutralization and or negativecharge storage into the floating gate. Both of these involves positivecharge neutralization and or negative charge storage into the floatinggate. Both of these mechanisms are expected to dominate during thewriting process resulting in net negative charge storage in the floatinggate and transition of the cell to the non-conducting high thresholdstate. Some loss of electrons from the silicon floating gate to thesilicon substrate defined as mechanism 440 is feasible at the initialtransient period when the field associated with the floating gate andthe substrate is maximum. But this mechanism is relatively weak due tothe low field (k<0.5) between the floating gate and the substrate. Thisis represented by an arrow of intermediate thickness. The remainingmechanism 442, as shown in FIG. 4, is insignificant due to the high holeenergy barrier of 4.7 ev and the low field across SiO₂—Ta₂O₅ between thesilicon substrate and the silicon floating gate. This is represented bya thinner arrow. Therefore, primary charge transport between the siliconfloating gate and the aluminum control gate is also maintained by thisinvention during the write process.

The band diagrams of FIGS. 2-4 are useful to show the retentionenhancement and read-disturb immunity of the nonvolatile device of thepresent invention. Typically, for a device design employing a powersupply of 2.5 V to 3.3 V (Vdd), the threshold differential between thestates 0 and 1 is around 2-3 V. For a worst case stack design of K=0.5,the built-in potential is less than 1.5 V and therefore the associatedinternal field due to the stored hole charge may not exceed 2.5 E6 V/CM.Hole emission is negligible since the hole barrier is high to both thefirst insulator region and also the second insulator region (3.1 ev toZrO₂ and 2.9 ev to Ta₂O₅). Additionally, electron emission from thesubstrate and from the control gate are negligible since the electronbarrier is high (3.2 ev from the substrate to the first insulator regionand 3.8 ev from the control gate to the second insulator region).Therefore, the charge retention for the nonvolatile device of thepresent invention is expected to be excellent.

With respect to the read-disturb immunity, a positive potential of Vddis applied to the control gate and the bit line while the substrate andthe source is held to the ground potential during a read pulse as shownin FIG. 7. For those word line bits at 0 state in which the floatinggate is storing holes and is providing a built-in potential, the peakfield may exceed 4E6 V/CM. However, lower mobility of holes and higherbarrier height would prevent any significant charge loss compared to aconventional design. On the other hand, for bits at 1 state, the fieldis reduced to less than 2.5 E6 V/CM. Therefore, electron emission andtunneling from the floating gate is expected to be negligible even witha relatively lower 1.5 ev barrier height from the floating gate to theZrO₂ of the second insulator region. As such, the read-disturb immunityfor the nonvolatile device of the present invention is expected to besignificantly enhanced.

FIGS. 5-7 generally illustrate the operating conditions for oneembodiment of the nonvolatile device of the present invention. Theequivalent oxide thickness (t_(ox.eq.)) of the gate stack isapproximately 10 nm, wherein the thickness of SiO₂ is approximately 2nm, the t_(ox.eq.) of Ta₂O₅ is approximately 4 nm, and the t_(ox.eq.) ofZrO₂ is approximately 4 nm. In this embodiment, the power supply voltageis approximately 2.5 V, the threshold voltage in the nonconducting state(VT(1)) is approximately 2.5-3.0 V and the threshold voltage in theconducting state (VT(0)) is approximately 0.5-1.0 V.

FIG. 5 illustrates operating conditions for erasing an NFET nonvolatiledevice of FIG. 1. Erasing the NFET nonvolatile device involves atransition from a nonconducting state 1 to a conducting state 0. Assuch, holes are to be transported into and/or electrons are to betransported off the floating gate to lower the threshold voltage fromVT(1) to VT(0). The drain voltage (Vdd), the source voltage (Vss), andthe substrate voltage (Vsx) are pulled to the ground potential. Apositive programming pulse is applied to the control gate as the controlgate voltage (Vcg). In one embodiment, the programming pulse isapproximately +12 V. In one embodiment, the +12 V programming pulse hasa duration approximately in the range of 1 to 10 msec. One of ordinaryskill in the art will understand that this programming pulse is capableof being produced from the 2.5 V power supply using charge pumps.

FIG. 6 illustrates operating conditions for writing an NFET nonvolatiledevice of FIG. 1. Writing the NFET nonvolatile device involves atransition from a conducting state 0 to a nonconducting state 1. Assuch, electrons are to be transported into and/or holes are to betransported off of the floating gate to raise the threshold voltage fromVT(0) to VT(1). The substrate voltage (Vsx) is allowed to float, and thesource voltage (Vss) and the control gate voltage (Vcg) are pulled tothe ground potential. A positive programming pulse is applied to thedrain to enable the substrate voltage (Vsx) to float up by capacitivecoupling. In one embodiment, the programming pulse is approximately +12V. In one embodiment, the duration of the +12 V programming pulse isapproximately 1 to 10 msec. One of ordinary skill in the art willunderstand that this programming pulse is capable of being produced fromthe 2.5 V power supply using charge pumps.

FIG. 7 illustrates operating conditions for reading an NFET nonvolatiledevice of FIG. 1. The power supply voltage (Vdd), e.g. 2.5 V, is appliedat both the drain node as well as at the control gate (Vcg). Thesubstrate voltage (Vsx) and the source voltage (Vss) are pulled to theground potential. Current (I) is sensed between the source and the drainto determine if the NFET nonvolatile device is in a conducting lowthreshold state 0 or in a nonconducting high threshold state 1.

FIG. 8 illustrates one embodiment of a floating plate embodiment of thenonvolatile device of the present invention. The device is similar tothat shown in FIG. 1, except that a floating plate replaces the floatinggate as the floating charge-storage region. As will be explained in moredetail below, the floating plate has silicon nano crystals that enhancefield emissions and is at least an order of magnitude thinner (<10 nm)than the thickness of the floating gate.

The device 800 is formed on a substrate 802. In one embodiment, thesubstrate includes p-silicon. A first source/drain region 804 and asecond source/drain region 806 are formed in the substrate betweenisolation regions 808. The source/drain regions are separated by achannel region 810 of the substrate. In one embodiment, each of thesource/drain regions include n−silicon source/drain extensionspositioned to define the channel region in the substrate, and n+siliconused to provide regions for a drain contact and a source contact.

An asymmetric gate stack 812 is formed over the channel region of thesubstrate. The gate stack separates a control gate 814 from thesubstrate. According to one embodiment, the control gate includesaluminum, and is connected to an aluminum word line. One embodiment ofthe asymmetric gate stack 812 includes a first insulator region 816, afloating plate 818 that functions as a floating charge-storage region,and a second insulator region 820. The gate stack is engineered toprovide the desired asymmetric barrier energies for charge transportbetween the three nodes (substrate, floating charge-storage region, andcontrol gate) of the device, and is further engineered to provide thedesired electric fields between the three nodes of the device.

In one embodiment of the asymmetric gate stack, the first insulatorregion 816 includes a layer of Silicon Dioxide (SiO₂) 822 disposed overthe channel region, and a layer of Tantalum Oxide (Ta₂O₅) 824 disposedover the layer of SiO₂. One embodiment of the layer of SiO₂ includesapproximately 2 nm of NH₃-treated SiO₂. Another embodiment of the layerof SiO₂ includes approximately 2 nm of NO-treated SiO₂. In oneembodiment, the layer of Ta₂O₅ has an equivalent oxide thickness(t_(ox.eq.)) of approximately 3 to 5 nm. In one embodiment, the layer ofTa₂O₅ has an equivalent oxide thickness (t_(ox.eq.)) of approximately 4nm. A layer of Zirconium Oxide (ZrO₂) 826 is disposed on the siliconfloating plate. In one embodiment, the layer of ZrO₂ has an equivalentoxide thickness (t_(ox.eq.)) of approximately 4 to 5 nm. AnOxide-Nitride-Oxide (ONO) sidewall 828 surrounds the asymmetric gatestack 812 and isolates the device from other integrated circuit devices.

One embodiment includes a thin layer of silicon-rich-oxide (SRO) as thefloating plate 818. Another embodiment includes a thin layer ofsilicon-rich-nitride (SRN) as the floating plate 818. The actualthickness of the floating plate is approximately between 3 nm to 10 nm.The equivalent oxide thickness (t_(ox.eq.)) of the floating plate isapproximately between 1.5-5 nm. In one embodiment, the floating platehas a thickness of about 5 nm and an equivalent oxide thickness (tox.eq)of about 2.5 nm. The characteristics of SRO and SRN will be described inmore detail below with respect to FIGS. 9-13.

As an overview, SRO and SRN contain silicon nano crystals that arecapable of storing charge. These silicon nano crystals are capable ofsimultaneously acting as charge trapping centers as well as chargeemission centers. The silicon nano crystals modulate the local potentialdistributions and enhance the field emission. For effective chargestorage, one embodiment includes a desired composition for SRO that hasa refractive index of 1.6, and a desired composition for SRN that has arefractive index of 2.2. Other embodiments employ a wider range ofcomposition of SRO or SRN. The refractive index of SRO and SRN isdeveloped below with respect to FIGS. 9-13. The enhanced field emissionsallow floating plate devices to be programmed at a significantly reducedaverage field of approximately 7E6 V/CM as compared to the floating gatedevice programming field of 12E6 V/CM. The endurance (write-erase cyclesbefore failure) of the device is enhanced by many orders of magnitude bythe lower effective programming field. The programming voltage for thefloating plate embodiment of FIG. 8 is +/−9 V for a total gate stackequivalent oxide thickness (tox.eq) of 13 nm including the SRO/SRNlayer; whereas the programming voltage for the floating gate embodimentof FIG. 1 is +/−12V.

FIGS. 9-13 show relevant properties of silicon rich insulators (SRI)from the standpoint of charge trapping and charge injection or emission.FIG. 9 is a graph showing refractive index of silicon-rich siliconnitride films versus SiH₂Cl₂/NH₃ flow rate ratio (R). This graph isprovided herein to illustrate the known relationship between the siliconamount and the refractive index. The graph indicates that the index ofrefraction increases linearly with increasing silicon content. As such,the index of refraction of the films can be used as an indication of thesilicon content of the films. The arrows on the graph illustrateexperimental ratios.

FIG. 10 is a graph showing current density versus applied field forsilicon-rich silicon nitride films having different percentages ofexcess silicon. The current density (J) is represented in amperes/cm²,and log J is plotted against the electric field E (volts/cm) for Si₃N₄layers having a SiH₂Cl₂/NH₃ flow rate ratio R of 0.1, 3, 5, 10, 15 and31. This graph is provided herein to illustrate the known relationshipbetween the amount of silicon and the conductivity of the film. The plotshows that the Si₃N₄ layers having small additions of silicon (R=3 and5) exhibit a relatively small conductivity increase over stoichiometricSi₃N₄. The plot further shows that increasing silicon content at orabove R=10 substantially increases or enhances the conductivity.

FIGS. 11 and 12 provide graphs that illustrate the known relationshipbetween the flatband shift and applied fields for films having varyingpercentages of excess silicon as represented by the SiH₂C12/NH₃ flowrate ratio R. FIG. 11 is a graph showing flatband shift versus time atan applied field of 4×10⁶ volts/cm for silicon-rich silicon nitridefilms having varying percentages of excess silicon. For R=3, theflatband shift is greater than the shifts produced by films having an Rof 0.1, 10 or 15. The film having an R of 10 provides a greater flatbandshift than a film having an R of 15. FIG. 12 is a graph showing flatbandshift versus time at an applied field of 7×10⁶ volts/cm for silicon-richsilicon nitride films having varying percentages of excess silicon. Theflatband shift produced by the R=3 film is even greater than that shownin FIG. 11, while the shifts produced by the R=10 and R=15 films do notchange as appreciably. FIGS. 11 and 12 are provided to illustrate thecharacteristics of a charge storing medium and a more conductive chargeinjector medium as further explained below.

The graphs of FIGS. 9-12, which were described above, indicate that atlow additional silicon content, silicon-rich Si₃N₄ films function as acharge storing medium as they exhibit appreciably enhanced trappingcharacteristics (as shown by the high flatband shifts at moderate andhigh applied electric fields in FIGS. 11 and 12, respectively) withoutexhibiting appreciably enhanced conductivity characteristics as shown inFIG. 9.

Silicon-rich silicon nitride films deposited at an R of 3 or 5 (for arefractive index of 2.10 and 2.17, respectively) will possess enhancedcharge storing characteristics. This family of silicon rich nitride isidentified herein as charge storing SRN or CS-SRN. In general,silicon-rich nitride films having an R greater than 0.1 and less than 10(or, more specifically, having an index of refraction betweenapproximately 2.10 and 2.30) will provide appreciably enhanced chargetrapping or charge storing properties without providing appreciablyenhanced charge conduction. This charge trapping is characteristic of acharge storing medium that can be used as a floating plate within a gatestack of a NV device as illustrated in FIG. 8.

Silicon-rich nitride films having an R greater than 10 (or, morespecifically, having an index of refraction greater than 2.3) arereferred to as an injector medium. Silicon nitride injectors arepreferred over silicon oxide injectors because of the superior hightemperature stability of the former material. Silicon readily diffuseswithin silicon oxide at elevated processing temperatures, which disruptsthe injection threshold by reducing the localized field distortions.However, even at higher processing temperature, silicon does not readilydiffuse within Si₃N₄. A silicon-rich Si₃N₄ (SRN) injector providesappreciably enhanced charge conductance without providing appreciablyenhanced charge trapping over stoichiometric Si₃N₄. This is illustratedin FIGS. 11 and 12, which shows progressively reduced flatband shiftsfor R=10 and R=15 with progressively increased conduction. The family ofmaterials acting as injector material is identified as “i-SRN.”

FIG. 13 is a graph showing apparent dielectric constant K versusrefractive index for both Silicon Rich Nitride (SRN) and Silicon RichOxide (SRO). The SRN and SRO plotted in this graph were provided using aLow Pressure Chemical Vapor Deposition (LPCVD) process. The SRO wasfabricated at approximately 680° C., and the fabricated structureincluded 100 Å oxide and 150 Å SRO. The SRN was fabricated atapproximately 770° C., and the fabricated structure included 45 Å oxideand 80 Å SRN. As shown in the graph, the dielectric constant of siliconis around 12. Materials with a higher K than silicon are conventionallytermed a high K material, and materials with a lower K than silicon areconventionally termed a low K material. SRN that has a refractive indexof 2.5 or greater and SRO that has a refractive index of 1.85 or greaterhave apparent dielectric constants that are greater than 12. InjectorSRI includes these high K SRO and high K SRN family of materials. In thepresent invention, injector SRI layers are employed in the gate stack tofurther lower the programming field and enhance endurance.

In one embodiment, the floating plate embodiment of FIG. 8 is modifiedby incorporating an additional thin layer of “injector” SRN (i.e. SRNhaving a refractive index greater than 2.5) between the aluminum controlgate and the ZrO₂. In another embodiment, the floating plate embodimentof FIG. 8 is modified by incorporating an additional thin layer of“injector” SRN between the SiO₂ and the Ta₂O₅. In another embodiment,the floating plate embodiment of FIG. 8 is modified by incorporating oneadditional thin layer of “injector” SRN between the aluminum controlgate and the ZrO₂, and another additional thin layer of “injector” SRNbetween the SiO₂ and the Ta₂O₅. In one embodiment, the thickness ofthese thin layers of injector SRN is approximately 2-3 nm. Thesestructures lower the programming field. This concept is generallydescribed and illustrated below in FIGS. 14-19. Applicant discusses thisconcept in detail in the U.S. patent application entitled “ScalableFlash/NV Structures and Devices With Extended Endurance,” Ser. No.09/944,985, filed on Aug. 30, 2001, which has been incorporated byreference into the present application.

FIG. 14 is a cross-section view of a conventional nonvolatile fieldeffect transistor (NV FET) device. The illustrated device is fabricatedon a silicon substrate 1412 such as a p silicon substrate or p-well inwhich case it is referred to as a source electrode (SE), and isseparated from other devices by the isolation trenches 1414. The device1410 further includes diffused regions that function as a drain region1416 and a source region 1418, such as the illustrated n+diffusedregions in the p substrate. A field effect transistor (FET) channel 1420is formed in the substrate between the drain and source regions. Asource contact 1422 is formed to electrically couple with the sourceregion 1418, and a bit contact 1424 is formed to electrically couplewith the drain region 1416. A floating polysilicon gate 1426 is formedover the FET channel 1420, and is separated from the FET channel 1420 bytunnel oxide 1428. A control gate 1430, referred to as a programelectrode (PE) for the illustrated embodiment, is formed over thefloating polysilicon gate 1426. An oxide/nitride/oxide (ONO) interpolydielectric 1432 is provided around and between the PE 1430 and thefloating gate 1426. A bit line is connected to the bit contact, and aword line is connected to the PE. An oxide 1433 is formed around the NVFET device.

Common dimensions for a typical NV FET device in the 0.13 to 0.15 μmtechnology generations are provided below. The cell size for a NAND gateis approximately 0.15 μm². The FET channel is approximately 150 nm wide.Both the floating gate and the PE are approximately 150 nm wide andabout 250 nm thick. The tunnel oxide separating the floating gate fromthe FET channel has a thickness 1529 of approximately 8 nm. The ONOinterpoly dielectric separating the PE and the floating gate isapproximately 15 nm thick. The programming voltage applied to the PE isabout 16 volts, and the pulse width of a programming pulse isapproximately 1 ms. The field generated across the tunnel oxide isapproximately 12×10⁶ V/cm. The minimum program window(V_(T)(“1”)−V_(T)(“0”)) is approximately 2 V. The minimum program windowis defined as the difference in the threshold voltages for a device witha stored one and a device with a stored zero. The endurance for atypical NV FET device is about 10₅ write/erase cycles. The power supplyV_(DD) is 3.3 V.

FIG. 15 illustrates the capacitive coupling for a conventional NV FETdevice. Again, the device 1510 includes a control gate or PE 1530, afloating gate 1526, and a substrate or SE 1512. A programming voltageVP₁ of 16 V is applied to the control gate. The electric field acrossabout 8 nm of tunnel oxide 1528 (E_(TUN.OX)) is approximately 12×10⁶V/cm, which reflects a coupling efficiency of about 60%. The lowefficiency is attributable to the geometry and capacitor divider effectsof the cell.

FIG. 16 illustrates the capacitive coupling for a nonvolatile floatingplate device. The device 1640 includes a control gate 1630 separatedfrom a substrate 1612 by a gate insulator stack 1642 having a thickness1629 of approximately 15 nm. The gate insulator stack 1642 includes atunnel insulator 1644, charge centers 1646 that form a floating platecapable of storing charge, and a charge blocking dielectric 1648. Aprogramming voltage VP₂ of 9.6 V is applied to the control gate 1630. Asthere is no separate floating gate, the coupling efficiency is 100%. Theaverage electric field E_(AVG) between the control gate 1630 and thesubstrate 1612 is between about 6 to 7×10⁶ V/cm.

FIG. 17 illustrates the average field enhancement due to theincorporation of a top injection layer in a gate stack for a nonvolatilefloating plate device. In this illustration, the gate insulator stack1740, which is interposed between the control gate 1730 and thesubstrate 1712, includes a tunnel layer 1750 that includes SiO₂ andTa₂O₅, a charge blocking layer 1752 that includes charge centers(CS-SRN) 1746 that form a floating plate or a charge storing medium, acharge blocking layer that includes ZrO₂, and an injector layer (i-SRN)1754. The injector layer (i-SRN) 1754 enhances the average electricfield by a factor of about 1.5 (1.5×) across the entire gate stack. Aprogramming voltage VP₃ of 5.5 to 6.5 V is applied to the control gate1730. The resulting average electric field E_(AVG) between the controlgate 1730 and the substrate 1712 is reduced to about 4×10⁶ V/cm.

FIG. 18 illustrates the average field enhancement due to theincorporation of a bottom injection layer in a gate stack for anonvolatile floating plate device. In this illustration, the gateinsulator stack 1840, which is interposed between the control gate 1830and the substrate 1812, includes an injector layer (i-SRN) 1856 overSiO₂, a tunnel layer of Ta₂O₅ 1850, and a charge blocking layer of ZrO₂1852 that includes charge centers (CS—SRN) 1846 to form a floating plateor a charge storing medium. In this embodiment, a programming voltageVP₃ of 5.5 to 6.5 V is applied to the control gate 1830. The resultingaverage electric field E_(AVG) between the control gate 1830 and thesubstrate 1812 is reduced to about 4×10⁶ V/cm. This illustrates that thesame general results are achieved whether the injector layer is on topof the gate insulator stack or on the bottom of the gate insulatorstack. That is, the injector layer (i-SRN) enhances the electric fieldby a factor of about 1.5×.

FIG. 19 illustrates the average field enhancement due to theincorporation of both a top injection layer (i-SRN) and a bottominjection layer (i-SRN) in a gate stack for a nonvolatile floating platedevice. In this illustration, the gate insulator stack 1940, which isinterposed between the control gate 1930 and the substrate 1912,includes a first injector layer SRN 1956 over a thin SiO₂ layer, atunnel layer of Ta₂O₅ 1950, a charge blocking layer of ZrO₂ 1952 thatincludes charge centers (CS-SRN) 1946 that form a floating plate or acharge storing medium, and a second injector layer (i-SRN) 1954 on topof ZrO₂. The use of an injector layer on the top and on the bottom ofthe gate insulator stack enhances the electric field by a factor ofabout 1.7 (1.7×). A programming voltage VP₃ of 5.5 to 6.5 V is appliedto the control gate 1930. The resulting average electric field E_(AVG)between the control gate 1930 and the substrate 1912 is reduced to about3.5×10⁶ V/cm.

Memory Array Level

FIG. 20 illustrates one memory array embodiment according to the presentinvention. The memory array includes at least one block of memory cells.The cells are arranged in rows and columns. All of the cells in aparticular column have drains D connected to a common bit line BL andall of the cells in a particular row have control gates connected to acommon word line WL. The bit lines BL are identified as BL0-BLM and theword lines WL are identified as WL0-WLN. The figure illustrates that allof the cells in a column have sources S connected to a common sourceline SLO, SL1, etc., but the invention is not so limited. Other sourceslines are capable of being used to connect to the source S of variouscells in the array. A single source line is capable of being connectedto the source S of a number of columns, and is capable of connecting thesources of all of the cells of the array to a ground potential.According to various embodiments, various numbers of cells are capableof being incorporated in a block or an array. The figure illustratesthat the backgate or substrate of all of the cells are connectedtogether via a backgate line (BGL). The invention is not limited to asingle backgate line as additional backgate lines may be used. Forexample, various backgate lines are capable of being used for arrays ofNFET memory devices that use p-wells to isolate the substrate ofselected cells from the substrate of other cells.

The cells are arranged in column pairs, with each cell of the pairsharing a common source S. For example, cell pair 2060 and 2062 have acommon source S connected to the source line SL. The drains D of thecells are connected to the bit line BL associated with the column inwhich the cells are located. For example, the cell pair 2060 and 2062have their drains D connected to a common bit line BL1.

One of the cells in the block of cells in the array is selectedaccording to address signals on address lines. The address signals aredecoded to select the lines that identify one cell. These lines used toidentify one cell include the word line and the bit line. Additionally,since more than one source line and/or backgate may be present, theappropriate source line and/or backgate line needed for programming canbe decoded from the address. A selected cell is capable of beingprogrammed (i.e. written/erased) and read.

A selected cell(s) is written (low to high threshold transition) bypulling the appropriate source line(s) and word line(s) to a groundpotential, raising the appropriate bit line(s) to +12 V by applying aprogramming pulse for approximately 10 msec., for example, while lettingthe substrate float.

A selected cell(s) is erased (high to low threshold transition) bypulling the appropriate source line(s) and bit line(s), as well as thesubstrate of the selected cells, to a ground potential, and applying apositive programming pulse +12 V for approximately 10 msec., forexample, to the appropriate word line(s).

A selected cell is read by pulling the appropriate source line, as wellas the substrate of the selected cell, to a ground potential, byapplying Vdd, such as a Vdd of approximately +2.5 V, to the appropriatebit line and word line, and by sensing the current on the bit line todetermine if the selected cell is in a conducting low threshold state 0or in a nonconducting high threshold state 1.

System Level

FIG. 21 is a simplified block diagram of a high-level organization of anelectronic system according to the teachings of the present invention.The system 2100 includes a memory device 2102 which has an array ofmemory cells 2104, address decoder 2106, row access circuitry 2108,column access circuitry 2110, control circuitry 2112 for controllingoperations, and input/output circuitry 2114. The memory device 2102further includes power circuitry 2116, a charge pump 2118 for providingthe higher-voltage programming pulses, and sensors 2120 such as currentsensors for determining whether a memory cell is in a low-thresholdconducting state or in a high-threshold nonconducting state. Also, asshown in FIG. 21, the system 2100 includes a processor 2122, or memorycontroller for memory accessing. The memory device 2102 receives controlsignals 2124 from the processor 2122 over wiring or metallization lines.The memory device 2102 is used to store data which is accessed via I/Olines. It will be appreciated by those skilled in the art thatadditional circuitry and control signals can be provided, and that thememory device 2102 has been simplified to help focus on the invention.At least one of the processor 2122 or memory device 2102 has a memorycell formed according to the embodiments of the present invention. Thatis, at least one of the processor 2122 or memory device 2102 includes anasymmetrical band gap engineered nonvolatile memory device that hasenhanced data retention and programming speed according to the teachingsof the present invention.

It will be understood that the embodiment shown in FIG. 21 illustratesan embodiment for electronic system circuitry in which the novel memorycells of the present invention are used. The illustration of system, asshown in FIG. 21, is intended to provide a general understanding of oneapplication for the structure and circuitry of the present invention,and is not intended to serve as a complete description of all theelements and features of an electronic system using the novel memorycell structures. Further, the invention is equally applicable to anysize and type of memory device using the novel memory cells of thepresent invention and is not intended to be limited to that describedabove. As one of ordinary skill in the art will understand, such anelectronic system can be fabricated in single-package processing units,or even on a single semiconductor chip, in order to reduce thecommunication time between the processor and the memory device.

Applications containing the novel memory cell of the present inventionas described in this disclosure include electronic systems for use inmemory modules, device drivers, power modules, communication modems,processor modules, and application-specific modules, and may includemultilayer, multichip modules. Such circuitry can further be asubcomponent of a variety of electronic systems, such as a clock, atelevision, a cell phone, a personal computer, an automobile, anindustrial control system, an aircraft, and others.

The figures presented and described in detail above are similarly usefulin describing the method aspects of the present subject matter. Theseaspects include, but are not limited to, methods of forming thenonvolatile memory device and arrays of nonvolatile memory devices, andmethods of operating the nonvolatile memory devices and arrays ofnonvolatile memory devices.

CONCLUSION

The present subject matter provides nonvolatile memory devices thatincorporate a band-gap engineered gate stack with asymmetric tunnelbarriers. The gate stack materials are selected with the appropriateenergy barrier between the silicon substrate, the floating gate/plate,the control gate, and the respective interface insulators. As such,field emission of carriers is selective to either electrons or holes,and carrier transport is selective between the floating node and thecontrol gate during programming. The selected carrier transport isenhanced between the control gate to the floating gate/plate due to thereduced barrier height at the control gate-insulator interface duringprogramming. Charges are retained in the floating gate/plate due torelatively higher barrier energy at the floating gate-insulatorinterface caused by the band-gap asymmetry. The insulator at the siliconsubstrate interface is selected so as to provide large barrier heightsfor both electrons and holes for minimal emission of carriers from thesilicon substrate during write or erase. Additionally, the insulatorthickness and stack geometry is designed to provide capacitive couplingK<0.5 to further limit carrier flow from and to the silicon substrate.In this manner, carrier flow is restricted primarily between the controlgate and the floating gate/plate in either direction for both write anderase.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. It is to be understood that the above description is intendedto be illustrative, and not restrictive. Combinations of the aboveembodiments, and other embodiments will be apparent to those of skill inthe art upon reviewing the above description. The scope of the inventionincludes any other applications in which the above structures andfabrication methods are used.

The scope of the invention should be determined with reference to theappended claims, along with the full scope of equivalents to which suchclaims are entitled.

1. A method for operating a plurality of adjacent nonvolatile memorycells, comprising: simultaneously erasing the plurality of nonvolatilememory cells from the high threshold nonconducting state (1) to the lowthreshold conducting state (0) by applying a positive programming pulseto a plurality of control gates of the plurality of adjacent nonvolatilememory cells while pulling a substrate common to all of the plurality ofadjacent nonvolatile memory cells to a ground potential, wherein thepositive programming pulse induces an erase primary charge transport ofholes from the control gate to a floating charge-storage region of eachof the cells; and simultaneously writing the plurality of nonvolatilememory cells from a low threshold conducting state (0) to a highthreshold nonconducting state (1) by tying the substrate common to allof the plurality of adjacent nonvolatile memory cells to a plurality ofdrain nodes of the plurality of adjacent nonvolatile memory cells andapplying a positive programming pulse to the drain nodes to capacitivelypull the substrate of the plurality of cells to a positive potentialwhile pulling the control gate of the cells to be written to a groundpotential, wherein the positive programming pulse induces a writeprimary charge transport of holes from the floating charge-storageregion of the plurality of cells to the control gate of the cells to bewritten.
 2. The method of claim 1, wherein both the positive programmingpulse applied at the control gates to erase the nonvolatile memory celland positive programming pulse applied at the drain nodes to write thenonvolatile memory cells includes a 10 msec. potential having a voltagecalculated to provide an electric field of approximately 12E6 V/cmbetween the control gate and the substrate.
 3. The method of claim 1,wherein both the positive programming pulse applied at the control gatesto erase the nonvolatile memory cells and positive programming pulseapplied at the drain nodes to write the nonvolatile memory cellsincludes a 10 msec. potential having a voltage calculated to provide anelectric field within a range of approximately 6E6 V/cm to approximately7E6 V/cm between the control gates and the substrate.
 4. The method ofclaim 1, wherein both the positive programming pulse applied at thecontrol gates to erase the nonvolatile memory cells and positiveprogramming pulse applied at the drain nodes to write the nonvolatilememory cells includes a 10 msec. potential having a voltage calculatedto provide an electric field of approximately 4E6 V/cm between thecontrol gate and the substrate, and wherein the positive programmingpulse induces a write primary charge transport of holes from thefloating charge-storage region of the plurality of cells through aninjector layer of silicon rich nitride having an index of refractiongreater than approximately 2.30 to the control gate of the cells.
 5. Themethod of claim 1, wherein both the positive programming pulse appliedat the control gates to erase the nonvolatile memory cells and positiveprogramming pulse applied at the drain nodes to write the nonvolatilememory cells includes a 10 msec. potential having a voltage calculatedto provide an electric field of approximately 3.5E6 V/cm between thecontrol gate and the substrate, and wherein the positive programmingpulse induces a write primary charge transport of holes from thefloating charge-storage region of the plurality of cells through aninjector layer of silicon rich nitride having an index of refractiongreater than approximately 2.30 to the control gate of the cells and acomplimentary charge transport of electrons from the control gate of thecells to the floating gate through the injector layer of silicon richnitride, and wherein the positive programming pulse induces a writesecondary charge transport of electrons from the floating charge-storageregion of the plurality of cells through a tunneling layer and throughan injector layer of silicon rich nitride having an index of refractiongreater than approximately 2.30 to an oxide layer between the floatingcharge-storage region and a channel region of the substrate of thecells.
 6. A method for operating a memory array of nonvolatile memorycells, where the memory cells are arranged in a first plurality of rowsand a second plurality of columns, each of the memory cells has at leasta drain region, a source region, a channel region, a floating gate, acontrol gate and a backgate, all of the control gates of the memorycells in at least one of the first plurality of rows are connected to acommon word line, all of the drain regions of the memory cells in atleast one of the second plurality of columns are connected to a commonbit line, all of the backgates of the memory cells in at least one ofthe first plurality of rows and in at least one of the second pluralityof columns are connected to a common backgate line, the methodcomprising: selecting at least one memory cell for erasing from the highthreshold nonconducting state (1) to the low threshold conducting state(0) by applying a positive programming pulse to a selected word linewhile source line, backgate line and bit line of the memory cells areconnected to a reference potential; selecting at least one memory cellfor writing from a low threshold conducting state (0) to a highthreshold nonconducting state (1) by applying a positive programmingpulse to a selected bit line while letting the backgate line float andconnecting a selected source line and word line to a referencepotential; and selecting at least one memory cell for reading byapplying a positive reading pulse to a selected bit line and word linewhile connecting a selected source line and the backgate line to areference potential, and sensing a current on the bit line to determineif the selected cell is in a conducting low threshold state (0) or in anonconducting high threshold state (1).
 7. The method of claim 6,wherein further the reading of the nonvolatile memory cell by applyingthe positive reading pulse comprises a pulse of approximately 2.5 V. 8.The method of claim 6, wherein further the reference voltage is a groundpotential.
 9. The method of claim 6, wherein both the positiveprogramming pulse applied to erase the nonvolatile memory array andpositive programming pulse applied to write the nonvolatile memory arrayincludes a 10 msec. potential having a voltage calculated to provide anelectric field of approximately 12E6 V/cm between the control gate andthe substrate.
 10. The method of claim 6, wherein both the positiveprogramming pulse applied to erase the nonvolatile memory array cellsand positive programming pulse applied to write the nonvolatile memoryarray cells includes a 10 msec. potential having a voltage calculated toprovide an electric field within a range of approximately 6E6 V/cm toapproximately 7E6 V/cm between the control gates and the substrate. 11.The method of claim 6, wherein both the positive programming pulseapplied to erase the nonvolatile memory array cells and positiveprogramming pulse applied to write the nonvolatile memory array cellsincludes a 10 msec. potential having a voltage calculated to provide anelectric field of approximately 4E6 V/cm between the control gate andthe backgate, and wherein the positive programming pulse induces a writeprimary charge transport of holes from the floating charge-storageregion of the plurality of cells through an injector layer of siliconrich nitride having an index of refraction greater than 2.30 to thecontrol gate of the cells.
 12. The method of claim 6, wherein both thepositive programming pulse applied to erase the nonvolatile memory arraycells and positive programming pulse applied to write the nonvolatilememory array cells includes a 10 msec. potential having a voltagecalculated to provide an electric field of approximately 3.5E6 V/cmbetween the control gate and the substrate, and wherein the positiveprogramming pulse induces a write primary charge transport of holes fromthe floating charge-storage region of the plurality of cells through aninjector layer of silicon rich nitride having an index of refractiongreater than 2.30 to the control gate of the cells, a complimentarycharge transport of electrons from the control gate of the cells to thefloating gate through the injector layer of silicon rich nitride, andwherein the positive programming pulse induces a write primary chargetransport of electrons from the floating charge-storage region of theplurality of cells through a tunneling layer and through an injectorlayer of silicon rich nitride having an index of refraction greater than2.30 to an oxide layer between the floating charge-storage region and achannel region of the substrate of the cells.
 13. The method of claim 6,further comprising reading the nonvolatile memory cell by applying thepositive reading pulse to a selected bit line and word line whileconnecting a selected source line and the substrate to a referencepotential, further comprises a reading pulse with a positive potentialof approximately 2.5 V.
 14. A method for operating a nonvolatile memorycell that includes a control gate and further includes a gate stackincluding a Silicon Dioxide (SiO₂) layer in contact with a channelregion of the memory cell; a Tantalum Oxide (Ta₂O₅) layer in contactwith the SiO₂ layer; a charge-storage region in contact with the Ta₂O₅layer, and a Zirconium Oxide (ZrO₂) layer in contact with thecharge-storage layer, the method comprising: erasing the nonvolatilememory cell from the high threshold nonconducting state to the lowthreshold conducting state, including applying a first programming pulseto the control gate of the cell while pulling a substrate of the cell toa first reference potential, wherein the first programming pulse inducesan erase primary charge transport of holes from the control gate throughthe ZrO₂ layer to the charge-storage region and a complimentary erasecharge transport of electrons from the charge-storage region through theZrO₂ layer to the control gate; writing the nonvolatile memory cell fromthe low threshold conducting state to the high threshold nonconductingstate, including applying a second programming pulse to a drain node ofthe cell to capacitively pull the substrate of the cell to a desiredpotential while pulling the control gate of the cell to a secondreference potential, wherein the second programming pulse induces awrite primary charge transport of holes from the charge-storage regionthrough the ZrO₂ layer to the control gate and a complimentary writecharge transport of electrons from the control gate through the ZrO₂layer to the charge-storage region; and reading the nonvolatile cell,including sensing drain current to determine if the memory cell is inthe low threshold conducting state or the high threshold nonconductingstate.
 15. The method of claim 14, wherein the first and secondreference potentials are the same potential, and the first and secondprogramming pulses are the same potential.
 16. The method of claim 15,wherein the first and second reference potentials are ground, and thefirst and second programming pulses are a positive potential.
 17. Themethod of claim 14, wherein both erasing and writing includes applying aprogramming field of approximately 12E6 V/cm across the gate stack. 18.The method of claim 14, wherein both erasing and writing includesapplying a programming field within a range between approximately 6E6V/cm and approximately 7E6 V/cm across the gate stack.
 19. The method ofclaim 14, wherein both erasing and writing includes applying aprogramming field of approximately 4E6 V/cm across the gate stack. 20.The method of claim 14, wherein both erasing and writing includesapplying a programming field of approximately 3.5E6 V/cm across the gatestack.